Respiratory monitoring

Monitoring a patient's respiratory status usually takes place in a hospital setting and may be the primary purpose for a patient being observed or admitted to a medical setting.

The physical signs of respiratory distress may present as a patient appearing short of breath, having an increased work of breathing, use of their accessory muscles, and changes in skin color, general pallor, or partial or complete loss of consciousness.

When the initial efforts of respiratory monitoring show evidence of a patient's inability to adequately oxygenate their blood, the patient may require mechanical ventilation.

It is key to have a good understanding of patient pathophysiology in order to properly interpret medical information.

Measurement of airway pressure (Paw), flow (F) and volume (Vol) during mechanical ventilation assists in the differential diagnosis of respiratory failure. Airway occlusion technique makes possible to carefully characterize the mechanics of the lung, chest wall, and the total respiratory system. Patients with acute respiratory distress syndrome (ARDS) can have a modified elastance due to a stiffer lung or a stiffer chest wall depending in the origin of the disease. Patients with ARDS of pulmonary origin are at greater risk of ventilator lung injury than those of non pulmonary origin.[1]

Recording muscle activity during spontaneous breathing helps differentiate PEEPi caused by dynamic hyperinflation from that caused by expiratory muscles. If the patients PEEPi is caused by dynamic hyperinflation, external PEEP will reduce the patient’s work of breathing. If it is caused by expiratory muscles, it will add an elastic load and it will increase the operating lung volume.

During a weaning trial, esophageal pressure and flow measurement can be used to partition patient’s effort into its resistive, elastic and PEEPi components. The three components are increased in patients that fail the weaning.

Presence of expiratory flow throughout expiration, without reaching zero, suggests the presence of PEEPi. With an occlusion of the expiratory port PEEPi can be measured in a patient in control ventilation.

Monitoring physiologic variables, such as the ratio of respiratory frequency to tidal volume (RR/VT) also called rapid shallow breathing index (RSBI) helps in deciding whether a patient has reasonably likelihood to tolerate discontinuation of mechanical ventilation.

Monitoring is essential with administration of therapeutic agents that can provide rapid and dramatic changes in a patient’s condition. For example, the measurement of airway resistance helps in assessing the response to bronchodilator therapy.

Monitoring of ventilated patients it is very important in all phases of ventilation. In control-ventilation it helps to reduce the potential risk of injuring the patient with the ventilation by better knowing the patients physiology. In assisted-ventilation it helps to set the ventilator to fulfill the patient demand. In weaning it helps to reduce the time needed to remove the ventilator from the patient.[2][3]

Tidal volume and respiratory rate are commonly used to set pressure-support ventilation. A reasonable level of inspiratory effort is an inspiratory pressure-time product (PTP) < 125 cmH2O.sec/minute. Pressure time product could be measured using the simultaneous recording of flow, volume and esophageal pressure. There are respiration monitors capable of measuring PTP on real time. If such a tool is not available to achieve this target, using a respiratory rate of 30 breaths/minute and tidal volume of 600mL resulted in the fewest false classifications.[5]

In a patient with acute lung injury (ALI), the right level of PEEP is that which optimizes arterial oxygenation without causing O2 toxicity or ventilatior-induced lung injury. Balancing the benefit of keeping the lung open (during tidal ventilation) against the risk of lung overinflation may require monitoring of the pressure-volume curve, lung morphology, and gas exchange. When loss of aeration has a focal distribution (atelectatic lower lobes coexisting with aerated upper lobes), a high level of PEEP can cause overinflation of already aerated areas and only partial recruitment of atelectatic areas. Different strategies exist to find the level of PEEP in these patients: ARDSnet,[6] guided by esophageal pressure,[7]Stress Index,[8] static airway pressure-volume curve.[9] In such patients, some experts recommend limiting PEEP to low levels (~10cmH2O). In patients who have diffused loss of aeration PEEP can be used provided it does not cause the plateau pressure to rise above the upper inflection point.

Work of breathing (WoB) is measured as the area of the pleural pressure – volume loop. Pleural pressure is assessed using esophageal pressure. The Campbell Diagram shows the different components of the WoB. Pressure Time Product (PTP), measures the patient respiratory effort, and has a higher correlation with oxygen consumption than WoB. During assist-control ventilation, an increase in flow can decrease work of breathing by as much as 60% in patients with acute respiratory failure. Higher flow rates can also decrease inspiratory effort in stable patients with COPD. During pressure support or assist-control ventilation, up to a third of patient effort may fail to trigger the ventilator. Such nontriggering has been shown to result from premature inspiratory efforts that are not sufficient to overcome the elastic recoil associated with dynamic hyperinflation. To trigger the ventilator, patient effort has to first generate a negative intrathorasic pressure to counterbalance the elastic recoil and then overcome the set sensitivity. The full consequences of wasted inspiratory efforts are not known. They certainly place an unnecessary burden on patients whose inspiratory muscles are already under stress. Such added stress can interfere with subsequent weaning.

The heterogeneous lung involvement in ARDS puts some regions at risk of developing alveolar overdistension when a ventilator breath is delivered. Plateau pressure is monitored as a surrogate for end-inspiratory alveolar pressure, and may help to minimize lung injury. Monitoring is key in early detection of hazardous situations. Mortality is four times higher when pneumothorax is not diagnosed immediately and treatment is delayed.

Pulse oximetry can provide an early warning of hypoxemia. An alarm on a ventilator may sound because of a change in ventilator performance or patient clinical status. An abrupt increase in peak airway pressure can arise with endothracheal obstruction or ventilator malfunction. A decrease in peak pressure can arise with a leak in the circuit. An increase in baseline airway pressure can signal malfunction of the exhalation valve.

Monitoring of physiologic variables over time helps in assessing a therapeutic response. Checking for trends assist in following the course of a disease. Monitoring patient effort can guide patient management during a weaning trial. Changes in esophageal pressure over the first 9 minutes of a trial, quantified as trend index, revealed sensitivity, specificity, positive predictive value, and negative predictive value.